Switched-mode power supplies are pervasive in the electronics field because of their ability to efficiently convert a first DC or AC voltage to a second regulated output level. Such power supplies are used, for example, in computer power supplies, DC power adapters, and automotive power supplies. As the demand for low power, low cost electronics have increased, however, a corresponding need for lower cost switched-mode power supplies have resulted.
One barrier to more efficient and cost effective switched-mode power supplies has been electromagnetic interference (EMI). High current switching in the tens to hundreds of KHz in a typical switched-mode-power supply has the propensity to create radio-frequency emissions that interfere with communication systems. In consumer and commercial electronics, these emissions are typically regulated by government bodies, for example, the Federal Communications Commission, that regulate and define maximum allowable EMI in particular frequency ranges. One way to reduce emissions is by using conductive shielding around the power supply. With the demand for light weight and inexpensive consumer electronics, adequate shielding is not always technically feasible or cost effective given a particular form factor and/or specification.
Another method of reducing emissions in a switched-mode power supply is to use a flyback converter architecture using a quasi-resonant (QR) controller. A QR controller reduces EMI by activating a switch within the power supply when the voltage across the switch is at a minimum voltage. By keeping the voltage across the switch at a minimum, EMI caused by the instantaneous sourcing or sinking of a large current is minimized.
In systems where the voltage across the switch approaches zero volts, for example, in low input voltage (e.g. Vin=230 Vac) power systems, EMI can be significantly reduced by tuning on the switch when the voltage across the switch is zero voltage. In high input voltage (e.g. Vin=230 Vac) power supply systems, however, the minimum voltage across the switch may still be appreciable during normal operation, which leads to increased EMI.
In the field of switched-mode power supplies, what is needed are cost effective, power efficient, and low EMI systems and methods for switched-mode power supplies.